A 22-year-old Arizona man has pleaded not guilty to federal charges of attempting to assassinate US Representative Gabrielle Giffords and attempting to murder two of her staff members.

Jared Lee Loughner is accused of opening fire on Ms Giffords and a crowd of bystanders outside a grocery store in north Tucson on 8 January, killing six people, including a federal judge, and wounding 13.

Ms Giffords was shot in the head but survived.